1. Field of the Invention
This invention relates to systems for extinguishing fires, and in particular to a system for adding liquid foam concentrate into water lines, and mixing the foam concentrate with water in a mixing device, such as a venturi and dispensing said mixture through a nozzle.
2. Description of the Prior Art
Conventional foam additive systems for fighting fires employ numerous mechanisms for supplying foam liquid concentrate via supply conduits to one or more of the discharge outlets of a water pump. The goal of such a system is to achieve xe2x80x9cbalanced pressurexe2x80x9d between the fluid line, typically a water line; and the additive line, typically a foam concentrate supply line. At balanced pressure, the system responds to high fluid pressure with a correlatively high additive pressure, and corresponds to low fluid pressure with a relatively low additive pressure. Thus, at high water pressure and flows, foam is added at an equal pressure and at a flow calculated to maintain a pre-determined ratio of water to foam. The same is true for low pressure and flow.
xe2x80x9cBalanced pressurexe2x80x9d is particularly important in the fire-fighting field, because the water to foam ratio is critical to optimize fire fighting efficiency based on the type of fire fuel that that is present. Ranges between 0.2%-6% of foam have been reported as optimal, depending on the composition and fuel of the target fire. Further complicating the task of balancing pressure is the extremely variable water flows and pressures. Thus, the amount and pressure of foam must meet the varying pressure and volume of water being used.
Various systems presently exist to attempt xe2x80x9cbalanced pressure.xe2x80x9d Early systems used the bladder method to add foam into the water line, such as described in U.S. Pat. No. 5,009,244 (1991) to Grindley et al. In a bladder system, a foam containing bladder is placed inside a pressurized water container, and the water line pressure is equilibrated with the air surrounding the foam bladder, keeping the foam and water at balanced pressure. However, this method severely limited the foam supply, such that realistically only one bladder could be used on each fire. Additionally, when the foam in the bladder was exhausted, bladder change-out was cumbersome and slow.
Another balanced pressure additive system involves an additive pump. These pump systems are of one or two basic types. One embodiment of an additive pump system is a bypass system, in which a foam pump runs at a constant speed discharging foam through a foam discharge outlet. A balanced pressure valve is located at the outlet and the balanced pressure valve allows the unneeded foam to re-circulate into a foam storage tank. However, this system responds poorly to water pressure variation in that it could not maintain exacting water to foam ratios. Another drawback of this system is that the foam tends to become heated as it is re-circulated into the foam storage tank. The heated foam, which has different flow characteristics than ambient temperature foam, mixes poorly with ambient temperature water. In addition the foam in the storage tank can become aerated due to the large quantity of foam being re-circulated. This aerated foam can cause the system to act erratic when it is drawn into the suction side of the foam pump.
Another embodiment of an additive pump system is a hydraulically powered demand system that varies additive pump output in response to different readings from a flow meter installed in the water pump discharge line that measures water flow rate. This system is disclosed in U.S. Pat. No. 5,174,383 (1992) to Haugen et al. The flow meter signal is processed by a microprocessor to match the output of the flow on the additive pump with a measure of the additive pump output fed back to the microprocessor to maintain the additive flow rate at the proper proportion to the water flow rate.
U.S. Pat. No. 4,436,487 (1984) to Purvis et al. teaches an improved system for supplying foam liquid concentrate to water pump discharge outlets wherein the output of the concentrate pump is controlled in accordance with the demand for liquid concentrate, irrespective of variations in water pump flow rate and operating pressure. This is achieved by driving the foam concentrate pump with a variable output hydraulic drive, which in turn is automatically modulated independently of the level of operation by the water pump by a hydraulic control circuit responsive both to the water pressure developed by the pump and to the foam liquid concentrate pressure developed by the concentrate pump. The hydraulic control circuit includes a fluid pressure responsive adjusting mechanism for varying the displacement of the hydrostatic pump. A servo control module is connected in the hydraulic control circuit between a rotary gear charge pump and the adjusting mechanism. The servo control module operates to modulate the hydraulic fluid pressure being applied to the adjusting mechanism in response to variations in the water pressure developed by the water pump and the foam liquid concentrate pressure developed by the concentrate pump. In other words, this system uses hydraulics to control the balanced pressure system. The servo control module senses the foam concentrate pressure and the water pressure. This in turn sends hydraulic fluid through a valve system that actuates a cylinder in the pump. The cylinder then mechanically changes the stroke of the hydraulic pump.
Another system is disclosed in U.S. Pat. No. 5,816,328 (1998) to Mason et al. To again maintain balanced pressure, a balances pressure valve with electric switches provides the required control. The system has two limit switches that are actuated when the shaft of the balanced pressure valve rises or falls. These switches start or stop an electric motor that with a cable that controls the stroke of the hydraulic pump. Because of the imprecise control, the balance valve is also capable of sending excess foam concentrate to the suction side of the foam pump for proper proportioning.
These prior art systems are either entirely mechanical in nature or electric components (i.e. switches and motors) are used to control the hydraulic pump. The present invention utilizes an electronic programmable logic controller to control the hydraulic pump.
The programmable controller provides the following advantages. The system has increased reliability due to the durability of circuit boards with no moving components. The system response time is increased due to advantages of electronics compared to mechanical methods. This response time is critical in this application due to the changes in flows and pressures at the fire scene. The accuracy of the system is critical with firefighter safety when using a class B foam blanket.
A basic objective of the present invention is to avoid the above-mentioned problems by monitoring fluid pressures not fluid flow, and electrically or electronically controlling foam additive flow. This solves a long-felt need for an accurate foam proportioning system based on demand. These and other objectives and advantages of the present invention are achieved in a preferred embodiment to be hereinafter described in greater detail. By contrast, the above-noted prior art devices rely upon fluid flow or hydraulic balancing circuits for maintaining balanced pressure. The prior art systems do not electronically control foam additive flow.
The foam additive system comprises a source of pressurized water, a water supply system in fluid communication with the source of pressurized water, and a fluid/additive mixing device in fluid communication with the water supply system. An additive pump, a motor, such as a piston motor mechanically coupled to the additive pump, and an additive supply line system connect the additive source with the fluid/additive mixing device. The additive supply line system is in fluid communication with the additive pump. An additive pump control apparatus is present for varying the rate of foam additive. The additive pump control apparatus is connected to the motor and the additive pump. The control apparatus is responsive to and in communication with a measure of additive supply line system pressure and a measure of water supply system pressure. Pressure transducers are utilized to measure the foam additive system pressure and the water supply system pressure.
The additive supply system further comprises a variable output hydraulic drive means coupled to and powering the motor. A hydraulic fluid reservoir supplies the hydraulic fluid required for the system. A hydrostatic pump having a pressure compensator and a swash plate is also provided, the hydrostatic pump being in fluid communication between the reservoir and the motor. In addition there is provided a second motor for driving the hydrostatic pump, the hydrostatic pump being operable to supply hydraulic fluid under pressure to the motor, and the hydrostatic pump having a variable displacement controlled by a pressure compensator.
The novelty or improvement of the present system includes an electronic proportional relief valve fluidly coupled to the hydrostatic pump and the hydraulic fluid reservoir and electronically connected to a digital controller. The aforementioned pressure transducers send electronic signals to the digital controller. The electronic proportional relief valve receives an electronic signal from the digital controller that in turn communicates a hydraulic pressure signal to the pressure compensator. In response to changes in the hydraulic pressure signal received from the proportional relief valve, the pressure compensator adjusts the swash plate. The compensator features the sensing ability to respond to pressure changes as quick as three variations per second. The compensator and an in-line orifice also serve as a speed control based on load sensing, preventing the additive pump""s hydraulic motor from exceeding a predetermined speed.
By using two electronic signals from the pressure transmitters, a method of electronically varying the additive supply system is also disclosed.